A conventional typical apparatus for forming a cross sectional image using ultrasound (or an image forming apparatus) has a transmitter portion that transmits ultrasound to a specimen (e.g. an object such as a living body to be inspected), a receiver portion that receives reflected ultrasound waves, scanning means for changing the direction of the transmitted and received ultrasound for scanning, and means for converting received reflected signals into brightness signals to visualize them. The interior of the specimen is observed using time-series cross sectional images obtained by the image forming apparatus having the above described configuration. In a type of such an apparatus, the direction of ultrasound is moved in the vertically and horizontally in a scanning manner by the aforementioned scanning means to form a three-dimensional image.
As described in patent document 1, when an ultrasound signal (which will be sometimes referred to simply as ultrasound) is transmitted or launched into a subject of inspection such as a living body, the ultrasound signal propagates in the subject and is reflected by a reflecting element(s) existing in the interior of the subject. Then, the reflected signal propagates in the subject of inspection again and is received by an ultrasound probe. In this process, there are phase differences between ultrasound waves when they reach to the ultrasound probe, since the tissue of the subject of inspection which serves as propagation medium is not homogeneous in general. Consequently, a phenomenon that an image formed from the received ultrasound waves is distorted (which is called a phase cancellation effect) occurs. The phase cancellation effect causes a distribution of delays of the reflected waves on the surface (which is sometimes also referred to as aperture or aperture surface) of the ultrasound probe from/on which the ultrasound signal is transmitted/received to deviate from theoretical values. For this reason, a significant improvement in the resolution cannot be achieved by simply increasing the aperture (aperture surface), and improvement in the image quality is difficult to achieve.
One method of solving this problem is phase conjugate transmission and reception. In the phase conjugate transmission and reception, a phase shaping addition process (which is sometimes also referred to as beam forming) is typically performed so that the directivity is automatically adjusted to the direction from which incident waves come based on the phase distribution on the aperture surface on which the incident waves are received. As a measure to solve the above described problem, this characteristic of the phase conjugate transmission and reception is used to achieve aberration correction rightly with respect to a target wave source (which is sometimes also referred to as an echo source) even in the case where the phase distribution on the incident wave receiving surface is distorted due to the sonic velocity distribution in the course of propagation. Specifically, two received wave signals are selected from among the wave signals received by a wave receiving array (which is sometimes also referred to as a receiving probe), and the inter-channel phase difference between the signals is determined. Thereafter, a non-ideal component contained in the inter-channel phase difference is obtained to determine a phase difference correction value. Then, the phase of the received wave signal is corrected using the phase correction value thus determined. The two received wave signals selected in the above process are two signals from each pair of adjacent elements among the ultrasound elements that constitute the wave receiving array, or alternatively the average of the signals from all the elements and a signal from each element. In the calculation of the phase correction value, cross-correlation calculation is used, as described in patent document 2. Here, the aforementioned “element” refers to a basic individual unit composed of one or plurality of ultrasound transducer(s) used to transmit and receive ultrasound.
Another method of aberration correction disclosed in patent document 3 includes a step of transmitting a first wave front from an array probe, and a step of determining an aberration correction value based on an obtained wavefront. This method further includes a step of transmitting a second wavefront reflecting the aberration correction value from the array probe, and a step of forming an image from an obtained second wave front. The first wavefront may be transmitted as a narrow beam in order to enhance accuracy of the phase, and the second wavefront may be transmitted as a wide beam in order to increase the scanning speed.
Non-patent document 1 discloses an attempt for enhancing accuracy of correction by repeatedly performing the method disclosed in patent document 1. According to methods disclosed in Patent documents 4 and 5, aberration correction is performed using one of the fundamental frequency and a harmonic frequency, while imaging is performed using the other.